Commonly, metal clips or clamps that engage embedded supports are used for securing steel railroad rails to concrete railroad ties, and non-conducting insulators are used to insulate the clips or clamps from the railroad rails. Metal clips or clamps of a type exemplified in Leeves U.S. Pat. No. 4,757,945, which when issued was assigned to Pandrol Limited of London, England, and metal clips or clamps of a type exemplified in Young U.S. Pat. No. 5,110,046, which when issued was assigned to McKay Australia Limited of Maidstone, Australia, are used widely in North America.
Commonly, when such clips are used, elastomeric pads are disposed between the lower flanges of the railroad rails and the railroad ties for cushioning the railroad rails and for insulating the rails electrically from the ties and from other underlying structures. Although ethylene vinyl acetate (EVA) rubber and other pads have been used widely for many years, polyurethane pads offering superior performance have become available commercially from ITW Irathane (a unit of Illinois Tool Works Inc.) of Hibbing, Minn., under its IRATHANE trademark.
Deterioration of the elastomeric pads and erosion of the concrete ties can occur if water infiltrates and freezes between the pads and the ties or if sand, which is used commonly to increase traction on grades, or debris infiltrates therebetween. Such deterioration and erosion problems can be quite severe, particularly under high loadings, in regions where weather conditions vary widely from summer to winter, at sharp curves, and at steep grades. Such deterioration and erosion problems can result in so-called "tie seat abrasion", which if severe can result in a railroad tie being judged unsafe for further service in a railroad track and having to be replaced.
Prior efforts to address such deterioration and erosion problems are disclosed in Buekett U.S. Pat. No. 4,925,094. As disclosed therein, a stainless steel or other non-corrodible metal or plastic plate is cast into an upper surface of a concrete tie. A rubber or plastic pad is interposed between the lower flange of a railroad rail and the plate that has been cast into the tie. The pad merely rests upon the plate and is free to move relative to the cast-in-place plate, held in place only by the clips.
Other efforts to address such deterioration and erosion problems are disclosed in Young U.S. Pat. No. 5,110,046. As disclosed therein, either an abrasion-resistant plate of an unspecified material is bonded to the upper surface of a concrete tie by an adhesive layer, epoxy resin adhesives being preferred, or a high density polyethylene (HDPE) closed cell foam is interposed between the abrasion-resistant plate and the upper surface of the concrete tie. In either instance, a rubber, polyurethane, or other elastomeric pad is interposed between the lower flange of a railroad rail and the upper surface of the concrete tie. Again the elastomeric pad merely rests upon the plate, held in place by clips, and is free to move relative to the adhesively bonded or foam-separated plate.
As a matter of related interest, Brown U.S. Pat. No. 5,261,599 discloses an elastomeric pad having resiliently deformable sealing portions, which are intended to form a watertight seal between the pad and the upper surface of a railroad tie, such as a concrete tie.
In U.S. patent application Ser. No. 08/333,522, supra, a composite pad is disclosed, which addresses such deterioration and erosion problems. The composite pad comprises an elastomeric pad, such as a polyurethane pad, to underlie the rail flange and a bonding member, such as a galvanized, organically coated, steel plate, or a rigid, polymeric film or sheet to overlie the railroad tie. An adhesive layer, for which a methacrylate ester composition is preferred, is employed for bonding the composite pad to the railroad tie. Preferably, a polyurethane pad is cast onto a steel plate, whereby the steel plate is bonded directly to the polyurethane pad. Because of differential shrinkage, however, it has been found that undesirable warpage of the composite plate can occur when a polyurethane pad is cast onto a steel plate or rigid, polymeric film or sheet.